Fix wxMotif compilation with g++ 4.4.

Fix wxMotif compilation with g++ 4.4.

wxMenu::CreateMenu() parameter called "index" was mistakenly wrapped in
WXUNUSED() making it invisible inside the function body and the (BSD string
function) index() was used instead. This somehow compiled before but not with
g++ 4.4. In any case, the code was wrong even when it did compile.

Remove WXUNUSED() from the parameter and also rename it to avoid clashes with
(semi-)standard function which can also be defined as macro on some systems.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64934 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775